Loughborough UniversityInstitutional Repository

Cure efficiency of dodecylsuccinic anhydride as across-linking agent for

elastomer blends based onepoxidized natural rubber

This item was submitted to Loughborough University's Institutional Repositoryby the/an author.

Citation: MASCIA, L. ... et al, 2015. Cure efficiency of dodecyl succinicanhydride as a cross-linking agent for elastomer blends based on epoxidizednatural rubber. Journal of Applied Polymer Science, 132 (6), 41448.

Additional Information:

• This is the peer reviewed version of the article, which has been publishedin final form at http://dx.doi.org/10.1002/app.41448. This article may beused for non-commercial purposes in accordance with Wiley Terms andConditions for Self-Archiving.

Metadata Record: https://dspace.lboro.ac.uk/2134/17399

Version: Accepted for publication

Publisher: c© Wiley Periodicals, Inc.

Rights: This work is made available according to the conditions of the Cre-ative Commons Attribution-NonCommercial-NoDerivatives 4.0 International(CC BY-NC-ND 4.0) licence. Full details of this licence are available at:https://creativecommons.org/licenses/by-nc-nd/4.0/

Please cite the published version.

1

CURE EFFICIENCY OF DODECYL SUCCINIC ANHYDRIDE AS A CROSS-LINKING AGENT FOR ELASTOMER BLENDS BASED ON EPOXIDIZED NATURAL RUBBER

Leno Mascia*a, Jane Clarke a, Kiat Seong Ng a, Kok Sien Chua a, Pietro Russob

a) Department of Materials, Loughborough University, Loughborough LE11 3QL, U.K b) Institute of Polymers, Composites and Biomaterials, National Research Council,

Via Campi Flegrei 34, 80078 Pozzuoli, Naples, Italy

*Corresponding author: L.Mascia@lboro.ac.uk

Abstract

Blends of a highly epoxidized natural rubber (ENR50) with unmodified natural rubber (NR)

and ethylene propylene elastomers (EPDM) were produced to evaluate the mixing and curing

characteristics. Dodecyl succinic anhydride was used to cross-link the ENR50 component and

the reactivity was assessed by monitoring the evolution of the torque in an oscillatory co-

axial cylinder rheometer, as well as by DSC thermal analysis. A physical model was used to

obtain a single parameter for the reactivity of the system, which corresponds to the rate

constant for first order curing reactions. Although the blends were thermodynamically

immiscible, displaying no significant change in Tg, the components were well dispersed at

microscopic level. Better mechanical properties were obtained for blends with EPDM.

Keywords: Epoxidized natural rubber, Blends, Anhydride cure, Natural rubber, Dodecyl succinic anhydride.

Introduction

Epoxidized natural rubber (ENR) has been known for some time and is commercially

available in three grades, respectively known as ENR10, ENR25 and ENR50, where the

integer denotes the mol% of epoxide groups incorporated into the backbone of the polymer

chains of natural rubber (NR) [1,2]. Epoxidation of NR can be carried out either in solution

2

or from the latex using specific amounts of peroxy formic acid to control the degree of

conversion [3-5]

The presence of epoxy groups in the chain increases the polarity, giving rise to

enhanced adhesion characteristics and oil resistance, but also to an increase in glass transition

temperature up to -24 oC for ENR50 [3,4]. The epoxy groups sited along the polymer chains

enhance the compatibility of ENR with polar polymers, such as polyvinyl chloride and

polyamides for the production of blends [6,7]. Ouhadi et al. have demonstrated that the

choice of vulcanizing agent can have a significant effect on properties (such as compression

set, oil resistance and the tensile strength) of a TPV system obtained from blends of

polypropylene (PP) and ethylene propylene diene monomer elastomer (EPDM) [8]. Since the

use of a peroxide initiator for chemically binding the two components gives rise also to two

simultaneous side reactions, i.e. crosslinking of EPDM phase and degradation of the PP

component the amount of peroxide added and processing conditions have to be carefully

controlled to obtain the desired reactions balance [7].

Halimatuddahliana et al. [9] have compared the properties of ternary blends of

PP/EPDM/NR and PP/EPDM/ENR, produced by conventional compounding and

vulcanization and have found that although the oil resistance of the PP/EPDM/ENR system

was better than that shown by PP/EPDM/NR blends, the latter system exhibited better tensile

properties.

Owing to the dual functionality of the epoxy groups in ENR, other researchers have

directed studies to the development of novel self-vulcanizable blends based on mixtures of

ENR with other functionalized polymers, such as carboxylated nitrile rubber and

chlorosulfonated polyethylene, omitting the use of conventional vulcanizing agents. [2,

12,13]. Pire et al [3,4] have produced self-vulcanizing rubber systems from ENR with

different epoxide content using dodecanedoic acid (DDDA, a dicarboxylic acid) as curing

agent. These authors have found an enormous acceleration effect on the cure rates by the

incorporation of an auxiliary agent, 1-2 dimethyl imidazole (DMI) at 2:1 molar ratio with

DDDA [4]. The effect was attributed to the formation of the corresponding ammonium

dicarboxylate salt, which can more readily open the oxirane ring than the corresponding acid.

In the present study the authors examine the use of dodecyl succinic anhydride

(DDSA) as a crosslinking agent for mixtures of ENR with NR and EPDM respectively, as

well as exploring the potentiality of ENR for the production of TPV-type systems, consisting

3

of heterogeneous mixtures containing a cross-linked elastomer as the main component and a

linear polymer to impart melt flow characteristics. At the same time, the pendant dodecyl

segments from the DDSA that will be present in the cross-links are expected to provide

internal plasticization, through a screening effect on polar groups within the chains, as well as

providing a compatibilization mechanism for blends with hydrocarbon elastomers. The

expected difference in the cross-links inherent to the two systems is illustrated in Figure 1.

Figure 1: Formation of crosslinks in ENR through the reaction of epoxy groups with DDSA (Present study - Left) and with the carboxylic groups in dodecanoic diacid (Pire et al. -Right) [3]

The high epoxy content grade ENR50 was used in the present study in order to achieve the

maximum possible reaction rates during processing from the available ENR systems. The

amount of anhydride used was set at 7 w%, which represents the estimated amount required

to nominally produce one cross-link per 100 carbon atoms along the polymer chain

(excluding the methyl group) at full esterification.

Experimental

4

Materials

a) Epoxidized Natural Rubber (ENR50, Epoxyprene 50, Mooney viscosity ML(1’+4’)100 oC

= 70 - 90, Manufactured by Muang Mai Guthrie Public Limited Company of Thailand) and Natural

Rubber (Standard Malaysian Rubber CV 60, were generously provided by Tun Abdul Razak

Research Centre.

b) (2-Dodecene-1-yl) succinic anhydride (DDSA), Xylene and N, N-Dimethylbenzylamine

(DMBA) were obtained from Sigma-Aldrich.

c) Precipitated silica, (Ultrasil VN2) was obtained from Evonik Degussa Industries.

d) EPDM, Vistalon 7500 (high molecular weight grade, 56 % ethylene and 5.7 % ethylidene

norborene) was obtained from Exxon Mobil Chemicals

Mixing procedure

The catalyst for the esterification reactions, N, N-Dimethylbenzylamine (DMBA),

was used at 2 parts per hundred parts (polymer + anhydride) in all cases. The DMBA was

premixed with amorphous silica, at 1: 0.7 weight ratio, to facilitate handling of this liquid

during the preparation of the blends.

50 g of TPV-type mixes were prepared in a Haake Rheomix laboratory internal mixer, fitted

with Banbury rotors, at 80 rpm in all cases. The mixes were obtained under two widely

different conditions, respectively 160 oC for 15 minutes (“Severe” thermal conditions) and 80

oC for 10 minutes (“Mild” thermal conditions). It should be noted the non-crosslinkable linear

polymer component in these mixes is another elastomer instead of a crystalline thermoplastic,

which is usually found in commercial products. This type of composition was deliberately

chosen to produce softer rubbers. Similar systems were studied by Wang et al. using EPDM

as the cross-linkable component and a poly(ethylene-co-octene) rubbery linear polymer [10].

The presence of dispersed cross-linked EPDM domains were found to provide considerable

improvements in the tensile strength and tear resistance over those exhibited by the pristine

poly(ethylene-co-octene).

It should be noted that in the present studiy based on ENR 50 as the vulcanizable component,

EPDM was used as the linear polymer component, instead of an EP copolymer, in order to

provide sites for additional cross-linking reactions by a free radical mechanism. These

reactions can be expected to take place simultaneously to the anhydride curing reactions, as

in the present case, or by a sequential enhanced vulcanization procedure that would make

5

use of a suitable peroxide initiator (not included in this study). The latter systems would offer

the possibility of tailoring the network of these types of rubber blends in such a way that one

phase can be more highly cross-linked than the other.

In the present study the mixes were produced as simple ENR/anhydride binary

compositions and also as blends with NR and EPDM at 25 and 50 w% levels with respect to

the total ENR/anhydride content. All components were mixed with the rotors set at the

required speed (80 rpm) by feeding the main polymer component first and allowing about 1

minute before lowering the ram to close the mixing chamber. The torque and temperature

were recorded continually and the data stored.

An additional ad hoc experiment was carried out to examine the likelihood of the formation

of cross-links through etherification reactions, catalyzed by the tertiary amine DMBA. For

this purpose 4 parts per hundred DMBA were added to the ENR50, (instead of 2 parts used in

the mixture with DDSA) and the mixing (at 160 oC) was extended to 20 minutes, as a means

enhancing the propensity of interactions between the epoxy groups in ENR50 and the tertiary

amine group in DMBA.

Curing evaluation

A commercial apparatus, known as the Wallace precision cure analyzer (abbreviated

to Wallace analyzer), was used to study the post-mixing cure characteristics of the blends by

monitoring the torque evolution with respect to time, at 1.7 Hz frequency and 0.44 strain

setting. This procedure was used to determine the residual reactivity of the pre-vulcanized

mixes as a means of estimating the additional curing requirements to complete the reactions.

The cure temperatures used were 180oC and 200oC and the tests were carried out for up to 30

minutes. These were deemed to represent limiting practical conditions for possible industrial

exploitation of the systems.

Approximately 2g samples were cut from the mix and placed in the chamber of the Wallace

analyzer. The evolution of torque with time was recorded automatically and the data were

stored for analysis.

The Wallace analyzer consists essentially of a combination of a coaxial cylinder

rheometer and a parallel disc oscillatory rheometer [13].

6

For the coaxial cylinder component the shear strain in the hoop direction is given by the

displacement gradient across the gap (Ro – Ri), i.e. γ = 𝑑𝑑𝑑𝑑

. When the gap is very small the

shear strain across the gap is constant so that 𝛾 = S/(Ro− Ri), where 𝑆 is the peripheral

displacement at the surface of the oscillating cylinder. The corresponding shear stress τ is

calculated from the torque T developed by the oscillatory cylinder to maintain the pre-set

angular displacement, using the equation τ = T/2π 𝑅𝑅3.

The shear strain and shear stress acting in the hoop direction in the parallel plate component

of the rheometer increase linearly from zero at the center to a maximum at the edge.

Reference values can be calculated from the peripheral displacement gradient and the

corresponding torque recorded in the measurements, i.e. γ(R) = S(R )/h= 2πΩR/h and τ(R)=

3T/2πR3, where S(R ) is the peripheral oscillation displacement, h is the gap distance, Ω is

the angular displacement of the spindle, T is the torque and R is the radius of the oscillating

disc.

Since the denominator terms of the equations used to calculate the shear stress are the same

for both components of the rheometer, resulting from Ro ≈ Ri ≈ R, the shear stress at the

edge of the parallel plate is equal to the value across the gap in the co-axial cylinder

component, hence no discontinuity is expected at in the variation of the stress at the

intersection. From the given equations it is estimated that the torque at the edge of the parallel

plate is one third the value across the gap of the co-axial cylinder. In view of the linear

decrease of the torque from the edge to the center of the parallel plate component the average

contribution to the total torque is effectively 1/6 of the torque for the coaxial cylinder.

Consequently the co-axial cylinder is the dominant component of the rheometer and,

therefore, for the purpose of the analysis the influence of the parallel plate can be considered

an “edge effect” that gives rise to overestimates of the shear stress acting across the gap. It is

well known that the reactions leading to gelation of low molecular weight thermosetting

systems are autocatalytic due to the cage effect on the mobility of the activated species while

approaching gelation conditions. In a recent study on the low temperature curing of systems

based on epoxy resins Lionetto et al. have shown that the increase in viscosity with time can

be described by the following equation [14]:

𝜂 = 𝜂0 𝑒𝜃𝜃 …………… 1

where η = viscosity at time t

7

η 0 = initial viscosity of resin mixture θ = reactivity parameter The applicability of such equation has been attributed to the equivalence of time and

temperature with respect to the activation energy required for the events leading to gelation.

However, the measurements on the systems of this work are performed to examine the curing

behavior after the onset of gelation and, therefore, the progress of the cross-linking reactions

that follows can be followed by monitoring the increase in modulus from an originally very

low value, i.e. tending to zero. The evolution of the torque after gelation can be described

using a time retardation function based on the premise that the formation of additional cross-

links during cure is retarded by the loss of mobility of the reactive chains and in concordance

with a similarly controlled retardation caused by the reduction in the concentration of reactive

species. Since the increase in modulus during curing is proportional to the rate of cross-

linking reactions, the process can be described by the following equation:

𝐺(𝜃) = 𝐺𝑅 + [𝐺∞ − 𝐺𝑅](1 − 𝑒−𝜃𝜃) …………………… 2

where G(t) = shear modulus at time t

G∞ = shear modulus at full reaction Θ = reactivity factor for curing reactions G0 = shear modulus at t = 0, where 𝐺∞ ≫ 𝐺𝑅, thus 𝐺𝑅 ≈ 0

This equation has the same basis as that used to describe the kinetics of 1st order reactions in

terms of the degree of conversion [15], expressed by

𝛼(𝑡) = 𝛼(∞)[1 − 𝑒−𝑘𝜃 ] ………………….. 3

where 𝛼 (𝑡) is the degree of conversion at time (t), 𝛼(∞) is the ultimate degree of

conversion and k is the rate constant for the reaction. Analogously, this equation also

assumes that 𝛼(∞) >> 𝛼(𝑅), the initial degree of conversion.

The equivalence of the equations 2 and 3 implies that any increase in the degree of

conversion brings about a proportional increase in the shear modulus, which results from

concomitant restrictions on the mobility of segments within polymer chains.

In view of the dynamic operating mode of the Wallace analyzer, that is to say that the

imposed signal follows a sinusoidal function, the modulus has to be taken to represent the

8

complex modulus G* . The increase in G* s expected to result from the larger increase in the

elastic component G’ over the decrease in the values for the loss component G.”

The above equation can be rewritten as follows:

𝐺∞ − 𝐺(𝑡)

𝐺∞−𝐺𝐺 = 1

𝑒𝜃𝑡 ………….. 4

𝐺∞−𝐺𝐺𝐺∞ − 𝐺(𝑡)

= 𝑒𝜃𝜃 …………… 5

As the shear modulus G is directly proportional to the torque (T), measured in the curing

experiments, a plot of log( 𝑇∞−𝑇𝐺𝑇∞−𝑇(𝑡)

) against ‘time’ would yield a linear relationship with the

gradient of the curve corresponding to the reactivity factor 𝜃2.303. The term T∞ refers to the

torque at the plateau (full reaction) in the torque-time curve, while T(t) is the torque at time t.

The advantage of using this procedure for the treatment of the raw torque data recorded by

the curometer, therefore, lies primarily on the possibility to obtain a single parameter for

comparing the reactivity of different systems.

It should be noted that, due to the alleviation of the errors related to the values of the torque

at numerator and denominator of the equation, the ratio ( 𝑇∞−𝑇𝐺𝑇∞−𝑇(𝑡)

) becomes very close to the

ratio of the corresponding shear modulus terms in equations 4 and 5.

Gel content measurements

Solvent extractions in boiling xylene were used to determine the gel content of the

blends after mixing. Since xylene is a suitable solvent also for dissolving all individual

components i.e. ENR, DDSA, EPDM and NR, it can be presumed that, in the absence of any

other extraneous cross-liking reactions, the resultant gel content only consists of insoluble

cross-linked DDSA/ENR, considering that the amounts of silica and accelerator used are

small and within the expected experimental error.

For each sample, approximately 0.2g to 0.3g of specimen were cut and weighed before

placing it into an aluminum (125 mesh) pouch of known weight. The specimens were then

9

directly immersed into about 150ml of xylene in a 250ml round bottom flask attached to a

condenser and heat was applied to boil the xylene for 2 hours. The pouches were taken out

and dried in a vacuum oven at 100oC for 3 hours and then weighed. Three measurements of

gel content for each sample were converted to w% fractions, using equation 6. In the case of

blends was also estimate the normalized gel content (relative to the amount of reactive

components).

𝐺𝑒𝐺 𝑐𝑅𝑐𝑡𝑒𝑐𝑡 (𝑤%) = 𝑊𝑒𝑊𝑊ℎ𝑡 𝑅𝑜 𝑟𝑒𝑟𝑊𝑟𝑟𝑒

𝑂𝑟𝑊𝑊𝑊𝑐𝑂𝐺 𝑤𝑒𝑊𝑊ℎ𝑡 𝑅𝑜 𝑟𝑂𝑠𝑠𝐺𝑒 × 100 … … … … … … … 6

Thermal analysis A Mettler Toledo DSC 1 apparatus was used in nitrogen atmosphere to produce

thermograms, respectively from room temperature to 240 oC and from – 50 to + 20 oC.

Measurements of tensile stress-strain relationship

Tensile tests were carried out using a laser equipped Houndsfield tensometer model

500L according to the ASTM D412 method. These tests were performed to assess primarily

the strain hardening characteristics of the systems examined, as the actual values for the

derived modulus and strength would not be particularly meaningful for systems that do not

contain all the required additives and auxiliaries that would be expected in a fully formulated

TPV system. Dumbbell shaped specimens with 27 mm of gauge length and 4 mm of width

were cut with a hydraulic press cutter from 1mm thick molded plaques prepared by

compression molding. (These were prepared at 160 oC in a hydraulic press using a 5 minutes

total heating/cure time). Tensile tests were performed at ambient conditions (21oC)

employing a crosshead speed of 100 mm/min up to fracture. From the recorded force and

extension the data were converted into nominal stress/strain curves, without allowing for

specimen thinning and the excess deviations from the requirements of rubber elasticity theory

for the calculation of strain.

Note that the gel content of the molded samples was not measured as the tensile tests were

carried out only on samples produced under “severe” thermal conditions, which had already

10

reached the maximum. Any further reaction taking place during pressing of the plaques

would be expected to have increased only the cross-linking density of the gel.

SEM Examinations

Examinations of the morphological structure of the various systems were made using

a Carl Zeiss (Leo/Cambridge) Stereoscan 360 scanning electron microscope (SEM) with an

operating voltage of 10kV. Compression molded specimens were cryogenically fractured

after immersion in liquid nitrogen and small pieces were mounted on aluminum stubs with

the fractured surface on the upper side and sputter-coated with a thin layer of gold.

Results and Discussion

Although the mixing characteristics of the systems examined were not a primary

concern of this study, it is worth reporting the appearance of a broad peak in the recorded

values of the torque upon reaching the steady state mixing regime. This feature can probably

be associated with the onset of the reactions between ENR 50 and DDSA and could be used

as a process control parameter for these systems.

Raw data for the evolution of cure

In Figure 2 are shown the traces representing the torque evolution for systems

produced under “mild” thermal conditions, monitored with the Wallace analyzer for curing

experiments at 180 oC (Right side) and at 200 oC (Left side). These indicate that only the runs

at 200 oC are quite close to reaching the equilibrium state before the end of the cure cycle,

however in both cases there is a clear evidence of the catalytic effect of DMBA on the curing

reactions. For the same systems produced under “severe” thermal conditions the traces reach

a plateau well within the 30 minutes run (see Figure 3).

11

Figure 2: Curing torque-time curves for 97ENR/7DDSA systems, mixed under “mild” thermal conditions. Left: Measurements at 200 oC. Right: Measurements at 180 oC . with DMBA, without DMBA.

Figure 3: Curing torque-time curve at 200 oC for 97ENR/7DDSA systems (with and without DMBA), both mixed under “severe” thermal condition.

The merging of the torque traces towards the end of the curing cycle in Figure 3 can be

explained by the results on thermal analysis (see Figure 6), which reveal the occurrence of

extensive degradation after the peak. Since at around 200 oC these exotherms are

considerably more pronounced for systems containing the DMBA catalyst, the flattening of

12

the torque traces at the upper end of the run in the Wallace analyzer can be associated with

the beginning of the breakdown of the network. Recent results from more detailed studies

currently carried out on these systems seem to indicate that the secondary exotherms arise

from the breakdown of the network through decarboxylation reactions (data to be reported in

the near future).

Analytical treatment of the curometer data

In all cases the initial torque (i.e. at t = 0), was assumed to be equal to zero since it is

difficult to estimate the precise values owing to the transient temperature regime in the initial

stages of the runs and to the lack of accuracy inherent to the instrument in this range. In

Figures 4 and 5 are shown plots of log( 𝑇∞−𝑇𝐺𝑇∞−𝑇(𝑡)

) against time obtained from rheometric cure

measurements at 200 oC relative to binary blends of 93ENR/7DDSA, both with and without

DMBA, mixed respectively under “severe” and “mild” thermal conditions.

Figure 4: Plot of 𝐥𝐥𝐥( 𝑻∞−𝑻𝑻𝑻∞−𝑻(𝒕)

) against time for tests at 200oC carried out on the two binary blends 93ENR/7DDSA (i.e. with and without DMBA) prepared under “severe” mixing conditions (160 oC for 15 minutes). (Ref. 16, Reproduced with permission).

13

The reactivity factor value, θ, for the ENR/DDSA systems mixed under “severe”

thermal conditions, obtained from the gradient of the straight line for the plots in Figure 4,

has been found to increase from 1.61 x 10-3 s-1 to 2.07 x 10-3 s-1 with the addition of the

DMBA catalyst. Note that the units for the reactivity factor may be taken to represent the

rate constant for the curing reactions, normalized with respect to the final state of cure.

Figure 5: Plot of 𝐥𝐥𝐥( 𝑻∞−𝑻𝑻𝑻∞−𝑻(𝒕)

) against time for tests at 200oC carried out on the two binary blends 93ENR/7DDSA (i.e. with and without DMBA) prepared under “mild” mixing conditions (80 oC for 10 minutes).

From the plots in Figure 5 it is observed that, except from the small deviation for data up to

about 300 seconds, there is a good fit to Equ. 5 also for the ENR/DDSA system mixed under

“mild” thermal conditions. The respective values for the reactivity factor, θ, increase from

0.92 x 10-3 s-1 to 1.15 10-3 s-1 for systems containing the DMBA catalyst, which confirm the

strong catalytic effect of this additive on the cross-linking reactions. In comparing the data

for the reactivity factor it is noted that, while the values are lower for mixes produced under

“mild” mixing conditions, the increment resulting from the addition of the DMBA catalyst is

approximately the same for both types of mixes. Hence, the higher values of the reactivity

factor for the two systems produced under “severe” thermal history has to be associated with

14

the more advanced state of the reactions at the beginning of the cure cycle, which are

expected to reach the final state at a faster rate. At the same time, it can be deduced that the

higher initial gradient of the plots in Figure 5 for systems produced under “mild” thermal

conditions, is related to the lower initial gel content of these mixes (see later), which may

display an autocatalytic behaviour before reaching full gelation, in accordance with Equ. 1

Curing evaluations by thermal analysis

In Figure 6 are shown DSC thermograms for systems produced under “severe” mixing

conditions with scans at 1 and 5 oC /minute. These display clearly the catalytic effect of the

DMBA additive on the curing reactions, evidenced in the form of sharp exothermic peaks

respectively at around 162 and 185 oC. For the non-catalyzed system only a small primary

endothermic peak can be identified for the run at the lower heating rate. Since the sharpness

of the peak is related to the reaction rate in the first stage of the reactions, consisting mainly

of ring opening and formation of monoesters, the absence of these features for non-catalyzed

system must be taken as a confirmation that the operating reaction mechanism relies on the

formation of the quaternary ammonium salt from the interaction of DDSA with DMBA, as

also evidenced in the work by Pire et al. [4].

15

Figure 6. DSC thermograms for samples produced under “severe” mixing conditions, obtained with scans at 1 and 5oC/min for both systems (i.e. with and without DMBA catalyst). (Ref. 16, Reproduced with permission).

The scans at the higher heating rates give rise to a small secondary exotherms in the region of

210 - 220 oC for both systems, which are indicative of additional exothermal reactions,

possibly consisting of a combination of cross-linking and chain scission reactions. These

features are presently addressed in a study aimed to clarify the chemical reactions involved

through ATR and NIR analysis, which have also revealed the occurrence of extensive side

reactions, particularly the formation of large quantities of vicinal diol groups in addition to

those derived from the opening of the oxirane ring by the reaction with the anhydride.

Gel content

The results have shown that the gel content was negligible for all individual

components; the highest recorded value was 1.01 w% gel for EPDM. Furthermore, the gel

content was also negligible (1.88 w%) for the particular ENR50 sample mixed only with

16

DMBA at 160 oC for 20 minutes, thereby excluding the possibility of etherification reactions

taking place prior to esterification reactions with the DDSA curing agent.

The measured gel content values and the related data for mixes produced under “severe”

thermal conditions are presented in Table 1. The maximum gel content recorded is about 88

w%. The discrepancy from 100 % can be attributed to the presence of non-functionalized NR

in the original ENR component used (as mentioned earlier).

Table 1: Gel content results for all mixes produced under “severe” thermal conditions based on ENR/DDSA vulcanizing system. (Note: All systems contain DMBA, except where otherwise stated)

Sample description Gel content

(w%)

Normalized Gel content

(w%)

Incremented Gel content

(w%) 93ENR/7DDSA 87.9 - - 93ENR/7DDSA (without DMBA) 83.8 - - 70ENR/5DDSA/25NR 70.4 93.9 5.9 46.5ENR/3.5DDSA/50NR 50.6 101.2 13.2 70ENR/5DDSA/25EPDM 69.2 92.3 4.3 46.5ENR/3.5DDSA/50EPDM 54.3 108.5 20.5

In examining the data in Table 1 it is noted that the normalized gel content of the blends is

higher than the value expected from the reactions between the ENR50 and DDSA. Therefore,

the incremented gel content (third column) has to be associated with the formation of

additional cross-links by a free radical mechanism. However, these become significant only

when the companion polymer is present at a high ratio with respect to the ENR50.

In comparison with the data in Table 1 the gel content for the mixes produced under “mild”

thermal conditions was slightly lower, falling within the range 81 – 84 %.

Mechanical properties

The nominal stress-strain curves for the binary ENR50/DDSA systems produced

under “severe” mixing conditions displayed a generally low elongation at break, while those

produced under “mild” thermal conditions exhibited low stress values without notable signs

of a strain hardening behaviour. A typical nominal stress – strain curve for the binary system

17

produced at 80 oC is shown in Figure 7. It should be noted that the molded plaques had a

“rather lumpy texture” owing to the high gel content of the blend at the end of the mixing

cycle, which limits the flow of the mix within the mold and provides an explanation also for

the the rippled nature of the stress/strain curve.

Figure 7: Nominal stress-strain curve for the catalysed ENR50/DDSA system prepared under “mild” thermal conditions (mixing at 80 oC for 10 minutes)

Better mechanical properties, particular with respect to strain-hardening characteristics, were

achieved for the ternary blends, as shown in Figure 8.

18

Figure 8: Nominal stress-strain curves for ternary systems derived from reactive mixtures of ENR/DDSA

As already indicated earlier the curves in Figures 7 and 8 for the stress-strain relationship

representing mechanical properties of the various compositions based on ENR/DDSA are to

be taken only as indicators of the mechanical behavior of the systems and are only useful to

assess their suitability for the possible development of related elastomeric products. On this

basis it is clear that blends with EPDM are displaying better characteristics than blends with

NR. However, it is important to note that the state of cure of the blends in the molded plaques

is far belowthe maximum level achievable for these systems and, therefore, the strength

values are generally lower than would be expected at full cure.

Thermal analysis at low temperatures

The DSC thermograms in Figure 9 indicate that the Tg for the uncured (as received) ENR50

is – 21.9 oC, while that the value for the sample prepared under “severe” mixing conditions

(gel content ~ 88 %) is – 19.0 oC, which is somewhat higher than the original elastomer and

suggests, therefore, that the use of DDSA curative does not induces the anticipated

plasticization expected from the screening effect of the dodecyl segments in the cured

elastomer. A small increase in Tg, however, is likely to arise purely as a result of the

introduction of crosslinks and, therefore, plasticization may be counteracted by

intermolecular attractions resulting from the formation of hughly polar groups, such as OH

and COOH (see Figure 1).

19

Figure 9. DSC thermograms at low temperatures for ENR50 (as received) and NR50/DDSA produced under “severe” mixing conditions (Ref. 16, Reproduced with permission).

Morphology

The SEM micrographs in Figure 10 show that the ENR/DDSA combination and

ENR/DDSA/NR blends containing up to 50 w% NR are well homogenized.

20

Figure10: SEM micrographs for the 93ENR/7DDSA system (a) and blends 46.5ENR/3.5DDSA/50NR (b), 69.75ENR/5.25DDSA/25EPDM (c) and 46.5ENR/3.5DDSA/50EPDM (d).

The blends with EPDM, appear to be slightly more heterogeneous but are well compatibilized,

as indicated in the micrograph taken at higher magnification for the system with 50% EPDM

content. The Tg values obtained by DSC thermal analysis for the 50 % blends are -16.0 oC

with EPDM and -16.5 oC with NR, which are slightly higher than the values obtained for the

corresponding ENR50/DDSA mixture. At the same time the Tg values indicate that neither

two uncross-linked elastomers is miscible with the ENR50, as these would have been

expected to reduce the Tg in view of the much lower inherent Tg values of both NR and

EPDM.

21

Conclusions

Mixtures of epoxidized natural rubber (ENR50) and dodecyl succinic anhydride

(DDSA), using molar amounts to produce one nominal cross-link for every 100 C atoms in

the backbone of the polymer chains at full esterification, were produced in a Haake Rheomix

laboratory internal mixer. For the mixing conditions used the gel content was found to be

within the range 81 – 88 w%. The curing behavior of the mixtures was monitored by

following the evolution of the torque in the oscillatory Wallace analyzer and the data were

fitted to a physical model that can be correlated with 1st process for the development of cross-

links. The reactivity factor, θ, which is synonymous with the rate constant for the curing

reactions, was obtained from plots of log( 𝑇∞−𝑇𝐺𝑇∞−𝑇(𝑡)

against time. Higher θ values were

obtained for mixes produced under “severe” thermal conditions than the corresponding

systems that received a “mild” thermal treatment during mixing. Irrespective of the thermal

conditions used in preparing the mixture, an increase of about 25 % in reactivity was revealed

by the addition of DBMA as a catalyst for the curing reactions. The DSC thermal analysis

for the residual reactivity of the systems after mixing, confirms the significant catalytic effect

exerted by the DBMA additive. Blends were also produced using different amounts of

natural rubber and EPDM respectively. Although a fairly high degree of homogeneity at

microscopic level was achieved for both systems, the blends were thermodynamically

immiscible. The blends with EPDM, however, displayed better mechanical properties.

Acknowledgements

This work has been partially supported by the POR Campania FSE 2007-2013 as part of the Materiali

e Strutture Intelligenti (MASTRI) Project. The authors are also grateful to Tun Abdul Razak Research

Centre for the generous donation of large quantities of ENR50.

References

1. X. Jiang, H. Huang, Y. Zhang, Y. Zhang, J. Appl. Polym. Sci., 92, 1437 (2004).

2. C. Nakason, S. Saiwari, J. Appl. Polym. Sci., 110, 4071 (2008).

3. M. Pire, S. Norvez, I. Iliopoulos, B.L. Rossignol, L. Leibler, Polymer, 51, 5903 (2010).

22

4. M. Pire, S. Norvez, I. Iliopoulos, B. L. Rossignol, L. Leibler, Polymer, 52, 5243 (2011).

5. T. Johnson, S. Thomas, Polymer, 40, 3223 (1999).

6. M. Narathichat, C. Kummerlöwe,N. Vennemann, K. Sahakaro, C. Nakason, Adv. Polym.

Tech., 31, 118 (2012).

7. C. Nakason, B. Thongnuanchan, K. Nokkaew, A. Kaesaman, Polym. Eng. Sci., 47, 421

(2007).

8. T. Ouhadi, S. Abdou-Sabet, H. Wussow, L.M. Ryan, L. Plummer, F. Baumann, J. Lohmar,

H.H. Vermeire, Ullmann’s Encyclopedia of industrial chemistry, Wiley-VCH Verlag GmbH

and Co, Published online in 2004, p 454-464.

9. H.I. Halimatuddahliana, Polym. – Plast. Techn. Eng., 43, 357 (2004).

10. Z. Wang, ,,X. Cheng,, J. Zhao, Materials Chemistry and Physics, 126, 272 (2011).

11. S. Mukhopadhyay, S.K. De, J. Appl. Polym. Sci., 42, 2773 (1991).

12. S. Toki, T. Fujimaki, M. Okuyama, Polymer, 41, 5423 (2000).

13. J. Butler, P.K. Freakley, Rubber Chem. Tech., 65, 374 (1992).

14. F. Lionetto, L. Mascia, M. Frigione, Eur. Polym. J., 49, 1298 (2013).

15. P. Musto, G. Ragosta, G. Scarinzi, L. Mascia, Polymer, 45, 1607 (2004).

16. L. Mascia, P. Russo, M. Lavorgna, L. Verdolotti, J. Clarke, A. Vignali, D. Acierno, AIP

Conf. Proc., 1599, 26-29 (2014): doi: 10.1063/1.4876769

23